TFPHTTPClient - frequent calls raise error
lazarus / 38-Networking and Web Programming

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ertank · 2019-12-12 15:30

Hello,

I am using Lazarus 2.0.7, SVN rev: 62300 from fixes 2.0 branch. fpc
version is 3.2.0 SVN rev: 43667 from fixes 3.2 branch. OS is Raspbian,
Debian 10.2 with all updates installed.

I have below function that I frequently call.
Code: [Select]function CallRestService(const URL: string; const Data:
RawUTF8; out Response: string): Boolean;
var
  Http: TFPHTTPClient;
  MS: TMemoryStream;
  ServerResponse: TMemoryStream;
  SS: TStringStream;
begin
  Log('REST: Calling URL: ' + URL);
  Log('REST Request: ' + string(Data));
  Http := nil;
  MS := nil;
  ServerResponse := nil;
  SS := nil;
  try
    MS := TMemoryStream.Create();
    MS.Write(Pointer(Data)^, Length(Data));
    MS.Position := 0;
    ServerResponse := TMemoryStream.Create();

    Http := TFPHTTPClient.Create(nil);
    Http.RequestHeaders.Values['Content-Type'] := 'application/json';
    Http.RequestHeaders.Values['UserAgent'] := EmptyStr;
    Http.ConnectTimeout := 60 * 1000;
    Http.IOTimeout := 60 * 1000;
    if UseProxy then
    begin
      Http.Proxy.Host := 'localhost';
      Http.Proxy.Port := 8888;
    end
    else
    begin
      Http.Proxy.Host := EmptyStr;
      Http.Proxy.Port := 0;
    end;

    try
      Http.RequestBody := MS;
      Http.Post(URL, ServerResponse);  // <--- EXCEPTION HERE
    except
      on E: Exception do
      begin
        Log('CallRestService()-Http.Post(): ' +
StringReplace(E.Message, sLineBreak, ' ', [rfReplaceAll]));
        Exit(False);
      end;
    end;

    SS := TStringStream.Create(EmptyStr, TEncoding.UTF8);
    ServerResponse.Position := 0;
    SS.LoadFromStream(ServerResponse);
    Response := SS.DataString;
    Log('REST Response: ' + Response);
    Result := True;
  finally
    SS.Free();
    ServerResponse.Free();
    MS.Free();
    Http.Free();
  end;
end;

When application is running without debug mode, my second call to it
raises exception "Project raised exception class 'ESocketError' with
message: Connect to <domain>:443 failed". My call times in log is as
following:
1- 2019-12-12 18:14:14.874
2- 2019-12-12 18:14:16.540


However, if I am to run application in debug mode, put a break point
on marked line in above code and hit F9 when code stops then
everything goes smooth. No exception, no error. My function call times
in debug run are as following:
1- 2019-12-12 18:23:41.796
2- 2019-12-12 18:23:47.452
3- 2019-12-12 18:23:51.208
4- 2019-12-12 18:23:53.944
5- 2019-12-12 18:23:56.544
6- 2019-12-12 18:23:58.496
7- 2019-12-12 18:24:00.911

All calls are to same exact domain from first to last. All calls are
using HTTPS protocol.

I failed to see what I am doing wrong and searching for help, please.

Thanks & regards,
Ertan

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trev · 2019-12-13 11:52

I had a similar issue a few years ago in a Delphi program. To cut a
long story short, the problem was being caused by the server's
firewall which was falsely detecting the quick succession of connects
as a denial of service attack. I only discovered the true cause
because I had access to the admin of the server in question. I believe
the firewall in question was Checkpoint.

Your  "ESocketError with message: Connect to <domain>:443 failed"
looked suspiciously similar to my experience at the time.

Just a thought.

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ertank · 2019-12-13 12:30

I am also suspecting about DDOS protection of that VPS I am
connecting.
Trying to figure...